It is common knowledge that petroleum products extracted from a well is stored on a floating storage facility or a sub sea tank. It is also known that it may be a Floating Production, Storage and Off loading vessel (commonly known as FPSO) or an Offshore Storage Unit (FSU) which are employed to temporarily store oil received from the well. These will in the following commonly be called storage vessel or simply vessel. Subsequently, it is transferred to an oil-tanker ship/vessel to bring it to the shore. Turrets are known to be employed for such transfer.
Some types of turrets are deployed at sea and the storage vessel has to position itself above the turret, and the turret is brought into a cavity beneath the vessel. When the turret is positioned within the cavity it is securely fixed to the vessel.
The turret that is used in connection with the present invention is installed in the vessel at the wharf, while the vessel is still in a dry dock. After the vessel has reached the offshore site where it is planned to be situated for receiving the hydrocarbons, the turret is connected to conduits from the oil or gas producing facility and also with mooring chains or wires to attach the turret to the seabed. Via the conduits petroleum products are transferred into the vessel. For that purpose, it is known that a riser pipe assembly and/or riser tubes, have to pass through the turret for ensuring transfer of the petroleum products to a receipt facility, which includes a swivel, located on the deck of the vessel.
To undertake the transfer operation without the risk of the vessel moving to an extent that could jeopardize the integrity of the conduits, the turret has to be properly moored to the sea bed, which naturally, ensures mooring of the vessel as well. Once the turret is moored to the sea, mooring forces act on the entire arrangement.
For safe transfer or the petroleum products, it is of utmost importance that the turret, the riser pipe assembly/riser tubes and the vessel should move vertically together. Further, it is also essential that the vessel, the riser pipe assembly/riser tubes and the turret should move horizontally together during the transfer, although this movement should be kept to a minimum. The vessel is allowed to rotate relative to the turret so that the vessel can be placed with its bow against the wind and/or waves.
The requirement stated in the preceding paragraph is essential to obtain optimal alignment and equilibrium in the whole arrangement. For that purpose, it is of utmost importance that the mooring forces are largely absorbed up by the turret and that other forces acting on the entire arrangement, should be suitably distributed and absorbed, so as to obtain optimal force distribution.
Patent publication WO 0151347 A1 discloses a rotating tower system for transferring hydrocarbons to a ship. It discloses a lower rotating body and an upper rotating body connected by an intermediate member. These two rotating bodies are independently mounted in relation to the ship/vessel. The lower and upper rotating bodies are each connected to the central portion of the intermediate member by respective articulated bearings for allowing angular deviations and for absorbing possible axial/radial forces acting on the arrangement during transfer.
WO 9917983 shows another turret arrangement having an inner tubular and an outer annular. All the risers are arranged in the annular part of the turret. It is not disclosed how the risers are hung off.
WO 2009141351 shows yet another turret arrangement. The risers are connected to some sort of manifold in the turret. There is no monobore in the turret.
EP 1433699 shows yet another turret arrangement. Here the turret is supported by an upper bearing above the deck of the ship. The riser package is supported vertically by the turret.
NO 19970071 also shows a turret arrangement where the risers are supported vertically by the turret.
The above prior art and also other prior art known in the field does not specifically teach how a turret can have features for connecting with the riser pipe assembly/riser tubes and with the vessel, so that the anchoring forces are largely absorbed by the turret, and so that the vertical hang-off forces from the riser package are absorbed by the ship independently of the turret.
In existing turret and riser systems, the turret and riser package are tailor-suited to one another. This means that when the production characteristics of the field changes, e.g., from a mainly oil producing field to a predominantly gas producing field, or if the same vessel is to receive hydrocarbons from a nearby field, the whole turret and riser arrangement may have to be changed in order to adapt to the new needs.
The present invention meets the needs mentioned in the preceding paragraph and other associated needs by providing a turret which is equipped with features for connection with the riser pipe assembly/riser tubes and for connection with the vessel so that optimal force distribution is achieved in the arrangement during petroleum transfer.